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From 1992 to 2000, eight former workers of the Gilster-Mary Lee popcorn plant in Jasper, Missouri, developed a rare lung disease called bronchiolitis obliterans. The disease was severe enough for some workers to cause them to be placed on lung transplant lists. In August 2000, the Missouri Department of Health asked NIOSH to determine if these cases of lung disease were due to an exposure at the plant, and if any current workers were at risk. After preliminary surveys were completed, NIOSH recommended that all workers in microwave popcorn production at the plant wear respiratory protection. In November 2000, NIOSH conducted voluntary tests of workers at the plant including confidential questionnaires, breathing tests, and chest x-rays. Current workers had chronic cough and shortness of breath 2.6 times as often as would be expected based on national surveys. Breathing tests showed abnormally low airflow 3.3 times as often as would be expected. About twice as many workers as would be expected had been told by their physicians that they had asthma or chronic bronchitis. From 1992 to 2000, eight former workers of the Gilster-Mary Lee popcorn plant in Jasper, Missouri, developed a rare lung disease called bronchiolitis obliterans. The disease was severe enough for some workers to cause them to be placed on lung transplant lists. In August 2000, the Missouri Department of Health asked NIOSH to determine if these cases of lung disease were due to an exposure at the plant, and if any current workers were at risk. After preliminary surveys were completed, NIOSH recommended that all workers in microwave popcorn production at the plant wear respiratory protection. The current data indicate that bronchiolitis obliterans occurring in former workers of this plant is most likely due to the workers' occupational exposure to inhaled flavoring vapors. Further research is needed to conclusively identify the cause and the circumstances under which the risk occurs. NIOSH is conducting surveys of other companies that use flavorings to see if other workers are at risk. These efforts will enable the development of guidelines for safe use of flavorings by all companies. The current investigation focuses on issues of worker safety. NIOSH is not aware of any evidence to suggest danger to consumers in the preparation and consumption of microwave popcorn.